Rootless Cone

A rootless cone is a volcanic landform that closely resembles a true volcanic crater but has no actual magma conduit connecting it to depth, meaning it is not itself a vent through which lava has erupted. It forms when a flow of hot lava crosses a wet surface such as a swamp, lake or pond, heating the water beneath it into steam; as pressure builds, the steam eventually bursts through the overlying lava in an explosion similar to a phreatic eruption, throwing up material that piles into a cone shaped mound. Because the resulting cone can look very much like a genuine volcanic crater despite having no root reaching into a magma source, rootless cones are a recognized source of confusion when mapping volcanic terrain.